To understand how one hormone regulates a target tissue's response to another hormone is the ultimate objective sought by this proposal. Specifically what is desired is an understanding of why prolactin is essential for the uteri of long-term ovariectomized rabbits to synthesize uterine proteins, as exemplified by uteroglobin, in response to progesterone. This goal will be achieved by measuring uteroglobin concentration in uterine fluids with immunodiffusion and RIA, to identify the optimal dosage and time of administration of prolactin and then to distinguish whether it functions in a "priming" capacity or acts synergistically in some other way with progesterone. The principal focus will be on prolactin induction of its own receptor and of estrogen and progesterone receptors and determining whether prolactin is acting on the same cells in the endometrium that synthesize uteroglobin. Receptor concentrations in both nuclear and cytosol fractions will be determined by Scatchard analysis of specific binding data and single saturating dose assay. Parallel protein and RNA synthesis will be measured by incorporation studies with 35S-methionine and 3H-uridine. Prolactin receptors will be localized immunohistochemically using a peroxidase anti-peroxidase method with monoclonal antibodies to prolactin and immunoglucose oxidase staining will be used to detect uteroglobin. The action of prolactin will be distinguished from the somatogenic action of growth hormone by comparing uteroglobin secretion, DNA synthesis, steroid receptor concentration and morphological changes in the endometria after single and combined treatment with the respective hormones and after inhibition of prolactin secretion by bromocryptine. The cellular level of prolactin regulation will be identified by the effect of inhibitors on transcription and translation in whole uteri in vitro and in a cell-free wheat germ lysate. In attempts to broaden the perspective and to improve the experimental system, studies will be made to induce uteroglobin synthesis in neonatal rabbits and cultured endometrial cells using prolactin, and the influence of prolactin on the progesterone-induction of other uterine proteins and on uteroglobin production in non-uterine tissues will be determined. If, in addition to its other roles in reproduction, prolactin serves the same function in the human species as noted here for the rabbit, namely to help formulate the progestational state of the uterus, then it is critical to good reproductive health. Its manipulation in a clinical context holds potential for regulating fertility.